New emissions legislation in Europe and North America is driving the implementation of new exhaust aftertreatment systems, particularly for lean-burn technologies such as compression-ignition (diesel) engines, and stratified-charge spark-ignited engines (usually with direct injection) that are operating under lean and ultra-lean conditions. Lean-burn engines exhibit high levels of nitrogen oxide emissions (NOx), that are difficult to treat in oxygen-rich exhaust environments characteristic of lean-burn combustion. Exhaust aftertreatment technologies are currently being developed that treat NOx under these conditions.
One of these technologies includes a catalyst that facilitates the reactions of ammonia (NH3) with the exhaust nitrogen oxides (NOx) to produce nitrogen (N2) and water (H2O). This technology is referred to as Selective Catalytic Reduction (SCR). Ammonia is difficult to handle in its pure form in the automotive environment, therefore it is customary with these systems to use a liquid aqueous urea solution, typically at a 32% concentration of urea (CO(NH2)2). The solution is referred to as AUS-32, and is also known under its commercial name of AdBlue. The urea is delivered to the hot exhaust stream typically through the use of an injector, and is transformed into ammonia prior to entry in the catalyst. More specifically, the urea is delivered to the hot exhaust stream and is transformed into ammonia in the exhaust after undergoing thermolysis, or thermal decomposition, into ammonia and isocyanic acid (HNCO). The isocyanic acid then undergoes a hydrolysis with the water present in the exhaust and is transformed into ammonia and carbon dioxide (CO2), the ammonia resulting from the thermolysis and the hydrolysis then undergoes a catalyzed reaction with the nitrogen oxides as described previously.
AUS-32, or AdBlue, has a freezing point of −11 C, and system freezing is expected to occur in cold climates. Since these fluids are aqueous, a volume expansion happens after the transition to the solid state upon freezing. This expanding ice can exert significant forces on any enclosed volumes, such as an injector, or fluid supply pipes. This expansion may cause damage to the injection unit, therefore, injection systems typically purge the injection unit when the engine shuts down to remove the fluid contained therein.
In the known system configurations, injector purging is used to remove fluid from the injector when the injector is not in use. It has been found that the efficiency of this method is not 100%, i.e., a certain amount of fluid remains in the injector unit. Although the amount of remaining fluid may not always be sufficient to risk damage to the injector upon freezing (expansion volume is available for the expanding ice), a risk remains that during engine hot soaks, the remaining fluid could be exposed to high temperature. This high temperature exposure could result in the decomposition of the AUS-32 which would also cause damage to the injection unit.
In other types of designs, it has been found that the remaining fluid tends to collect in the upper portion of the injector, in the volume between the filter and the inlet tube. Many types of injectors have O-rings which are used in combination with an injector cup to provide a sealing function, and prevent the remaining fluid from leaking. However, in some injectors, there is a potential leak path for the AUS-32 past the installed O-ring which cooperates with the injector cup to provide a sealing function. Although this sealing path created by the 0-ring is typically sufficient for liquids, it has been found that AUS-32 solution is prone to breaching seals of this type in the form of creeping urea crystals. At the fluid boundary layer, if there has been a minimal bypass of the sealing joint, fluid evaporates and leaves behind urea in its solid form. This provides a wicking path for more liquid urea solution, which establishes another boundary layer, evaporates, and leaves behind more solid urea. This creeping mechanism has often been observed on AUS-32 systems.
Accordingly, there exists a need for a way to purge an RDU, thereby sufficiently remove fluid from the RDU, and reduce or prevent the creeping mechanism as described above.